In presses, rolls are used in inking and damping units. Rolls normally comprise a roll core, for example in the form of a steel tube, and a functional layer on the steel tube. This functional layer can be a resilient rubber layer, a plastic layer with good ink transfer properties or ceramic or metallic layers with specific properties, such as hydrophilicity or wear resistance. Moreover, in offset presses, transfer cylinders (what are known as offset cylinders) are needed with which the images to be printed are picked up from an inked form cylinder in order to be applied to a printing material in contact with the transfer cylinder. Transfer cylinders of this type are normally called rubber-covered cylinders.
These rubber-covered cylinders likewise comprise a metallic cylinder core. This cylinder core can consist of steel and can be completely cylindrical or, in particular when rubber-coated sleeves are used, can be a hollow cylinder (i.e., also a metallic tubular cylindrical rotational body). A hollow cylinder can also be filled with a “foam” material.
At high printing speeds (e.g., high press rotational speeds of above 50,000 or even 70,000 revolutions/hour), heating becomes an issue, in particular in the rubber-covered cylinders in offset presses, and has very detrimental consequences. This heating is often non-uniform and leads to thermal expansions. These thermal expansions are exhibited as a deflection of the cylinder that is particularly caused by the temperature difference in the cross-section of the cylinder or the roll.
The deflections, which can be in the form of banana-like bulging of the rolls or cylinders, can lead to problems in the media transfer, for example during the transfer of the image to be printed from a rubber-covered cylinder to a printing material. The cause of non-uniform heating in the cylinder or the roll can lie in deviations from a circular shape or imbalances of the roll, which can never be completely avoided in the fabrication process. This effect is particularly noticeable in presses with very narrow rolls or cylinders and a relatively great length (i.e., press width). High rotational speeds exacerbate this effect.
In order to avoid this effect and its consequences, the rolls or cylinders previously have been cooled. To this end, liquid cooling media, water in the simplest case, had to be led through the cylinder through corresponding bores or tubular guides. The aforementioned problems can be partially eliminated via cooling. Cooling achieves an overall reduction in the temperature in the roll or the cylinder. However, local temperature differences cannot be completely avoided even with such a temperature reduction and ultimately could lead to the aforementioned problems. In addition, roll or cylinder cooling is complicated in design terms and is relatively costly.
Producing cylinders or rolls or roll cores from invar steel or from CRP is also already known. These materials exhibit very low or no thermal expansions, but are very expensive and, to some extent, cannot be easily coated with the typical functional layers.
In addition, the non-uniform heating in the rolls and cylinders can also depend on the particular use for the roll or cylinder. In particular, the distribution of ink and water on the cylinder surfaces play a part.